That just means that between x = 3 and x = 8 (including 3 and 8)
You can use
y = 3 x
if x were 4, you could say y = 3*4 = 12
BUT
if x is 9 , you are not allowed to use this relation and have to use some other one.
In this particular case, you would use
y = 3 - 5x
if x were 9, which is greater than 8